Cored steam turbine bucket

ABSTRACT

Steam turbine buckets are formed by investment casting to be hollow or to have a lightweight lattice-work internal core. The process is also used to integrally form steam turbine buckets having a variety of dovetail configurations, including a tangential fir-tree dovetail, a tangential slot dovetail and an axial gas turbine style dovetail. The investment casting process can also be used to integrally cast a cover at one end of the steam turbine bucket.

FIELD OF THE INVENTION

[0001] The present invention is directed to the investment casting ofsteam turbine buckets. More particularly, the present invention providesa method for investment casting lightweight yet structurally sound steamturbine buckets.

BACKGROUND OF THE INVENTION

[0002] Current steam turbine buckets are machined from plate stock andas such are solid buckets. This includes steam turbine buckets withtraditional tangential fir-tree dovetails, tangential slot dovetails,and axial entry dovetails.

[0003] A critical design element of steam turbine design is rotorstresses driven by bucket weight. With the given manufacturingprocesses, however, it is very difficult to reduce bucket weight withoutcompromising mechanical integrity.

[0004] The prior art technology for high pressure and intermediatepressure steam turbine rotating bucket airfoils is primarily customcomponents machined from plate stock. These solid airfoils arenecessarily heavy and necessitate significant rotor structures tosupport them.

SUMMARY OF THE INVENTION

[0005] It is desirable to replace the conventionally machined solidbuckets with lighter parts manufactured by investment casting withoutcompromising structural integrity. The present invention accomplishesthe goal of providing a lightweight yet strong, steam turbine airfoil.

[0006] The present invention proposes a investment casting process forhollow steam turbine buckets, which may also include a lattice-work ofcrossing channels internal to the airfoil to reduce weight whilemaintaining mechanical integrity. The new structural design uses arobust investment casting, facilitates an integral cover (or bucketshroud) design and provides the potential for steam turbine bucketcooling.

[0007] The lattice-work of channels is placed at opposing angles tocreate a cris-crossed rib structure internal to the airfoil with goodstress distribution. Preferably, there are no internal main ribs in thedesign, but could be if mechanical design considerations required suchmain ribs.

[0008] The lattice-work of channels is formed from a highly connectedceramic core piece, which leads to very high yield rates for investmentcastings. The new structural design results in an airfoil having a loweroverall weight, which serves to reduce corresponding rotor structuresand overall unit size.

[0009] Advantages of the new structural design include weight reductionand an improved rotor thermal transient response due to reduced weight.In addition, investment cast procedures facilitate design features suchas integral covers. Moreover, the inventive process provides the abilityto tailor design response by altering internal geometry (latticedensity).

[0010] The use of investment casting provides a high casting yieldrelative to traditional gas turbine designs. Finally, the use ofinvestment casting provides the potential for steam turbine bucketcooling.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011]FIG. 1 shows a cut-away section of a steam turbine bucket airfoilto have a lattice-work internal core and with the bucket having atangential fir-tree dovetail;

[0012]FIG. 2 shows a cut-away section of a steam turbine bucket airfoilto have a lattice-work internal core and with the bucket having atangential slot dovetail;

[0013]FIG. 3 shows a cut-away section of a steam turbine bucket airfoilto have a lattice-work internal core and with the bucket having an axialentry dovetail; and

DETAILED DESCRIPTION OF THE INVENTION

[0014] Due to the custom nature of conventional steam turbine designs,steam turbine buckets are traditionally manufactured from plate stockusing flexible machining processes. The present inventors were the firstto realize that as steam turbine products become more structured orpre-engineered, as are gas turbine products, the re-use of steam turbinebucket designs made an investment casting manufacturing process morefeasible.

[0015] They were also the first to realize that while investment castingprovided an opportunity for significant bucket weight reduction, thefragile nature of traditional gas turbine cores with a few internalradial ribs, often lead to a high rate of ceramic core breakage andvariability in cast wall thickness. The proposed investment cast processaddresses these problems and issues.

[0016] FIGS. 1-3 show in cut-away section, the use of lattice-works 10,20, 30 that respectively provide high solidity ceramic cores which aremuch more durable for manufacturing. Alternatively, these steam turbinebuckets could be formed by investment casting without the lattice-workinternal cores so as to be hollow.

[0017] The lattice-works 10, 20, 30, however, allow greater control ofwall thickness in the end product, and improve the mechanical integrityrelative to the traditional alternative with large internal cavities.FIG. 1 shows a steam turbine bucket having a tangential fir-treedovetail 11, FIG. 2 shows a steam turbine bucket having a tangentialslot dovetail 21, FIG. 3 shows a steam turbine bucket having an axialentry dovetail.

[0018] Preferably, the range of angles for the lattice work crossingchannels or ribs is from 25° to 55° on one wall and then −25° to −55° onthe opposite wall. The preferred ratio for size of the channels to thethickness of the ribs is 4:1 to 1:1. For example, for the midpoint ofthe range, ribs 0.1 inch would be spaced 0.25 inches apart.

[0019] The investment casting process also provides the opportunity tocast-in design features such as integral covers 12, 22, 32 that reducebucket tip steampath leakage. The reduced weight of the cast buckets andassociated rotor improve thermal transient response to be moreconsistent with the static shell structures, improving transientresponse, maintaining tighter clearances and improved unit performance.

[0020] In a structured steam turbine product line HP and IP buckets willbe re-used from unit to unit as steampaths are duplicated. This re-usemakes casting tooling and casting buckets as a manufacturing processfeasible. Casting opens the opportunity to significantly reduce bucketweight and the associated rotor mass by the use of cores.

[0021] While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

What is claimed is:
 1. A method of forming a steam turbine bucketairfoil by using an investment casting process and forming a hollowsteam turbine bucket airfoil.
 2. The method as claimed in claim 1,wherein a cover is integrally cast with the hollow steam turbine bucketairfoil.
 3. The method as claimed in claim 1, wherein the formed hollowsteam turbine bucket airfoil is integrally cast with a tangentialfir-tree dovetail.
 4. The method as claimed in claim 1, wherein theformed hollow steam turbine bucket airfoil is integrally cast with atangential slot dovetail.
 5. The method as claimed in claim 1, whereinthe formed hollow steam turbine bucket airfoil is integrally cast withan axial entry dovetail.
 6. The method as claimed in claim 2, whereinthe formed hollow steam turbine bucket airfoil is integrally cast with atangential fir-tree dovetail.
 7. The method as claimed in claim 2,wherein the formed hollow steam turbine bucket airfoil is integrallycast with a tangential slot dovetail.
 8. The method as claimed in claim2, wherein the formed hollow steam turbine bucket airfoil is integrallycast with an axial entry dovetail. 9 A method of forming a steam turbinebucket airfoil, said method comprising utilizing an investment castingprocess to form an internal core of the steam turbine bucket airfoilhaving a lattice-work.
 10. The method as claimed in claim 9, wherein acover is integrally cast with the steam turbine bucket airfoil having alattice work.
 11. The method as claimed in claim 9, wherein the formedsteam turbine bucket airfoil is integrally cast with a tangentialfir-tree dovetail.
 12. The method as claimed in claim 9, wherein theformed steam turbine bucket airfoil is integrally cast with a tangentialslot dovetail.
 13. The method as claimed in claim 9, wherein the formedsteam turbine bucket airfoil is integrally cast with an axial entrydovetail.
 14. The method as claimed in claim 10, wherein the formedsteam turbine bucket airfoil is integrally cast with a tangentialfir-tree dovetail.
 15. The method as claimed in claim 10, wherein theformed steam turbine bucket airfoil is integrally cast with a tangentialslot dovetail.
 16. The method as claimed in claim 10, wherein the formedsteam turbine bucket airfoil is integrally cast with an axial entrydovetail.
 17. A steam turbine bucket airfoil for use in a steam turbinecomprising a lattice-work internal core.
 18. The steam turbine bucketairfoil as claimed in claim 17, further comprising a cover formed at oneend.
 19. The steam turbine bucket airfoil claimed in claim 17, furthercomprising a tangential fir-tree dovetail.
 20. The steam turbine bucketairfoil claimed in claim 17, further comprising a tangential slotdovetail.
 21. The steam turbine bucket airfoil claimed in claim 17,further comprising an axial entry dovetail.
 22. The steam turbine bucketairfoil claimed in claim 18, further comprising a tangential fir-treedovetail.
 23. The steam turbine bucket airfoil claimed in claim 18,further comprising a tangential slot dovetail.
 24. The steam turbinebucket airfoil claimed in claim 18, further comprising an axial entrydovetail.
 25. A steam turbine bucket airfoil for use in a steam turbinecomprising a hollow airfoil core.
 26. The steam turbine bucket airfoilas claimed in claim 25, further comprising a cover formed at one end.27. The steam turbine bucket airfoil claimed in claim 25, furthercomprising a tangential fir-tree dovetail.
 28. The steam turbine bucketairfoil claimed in claim 25, further comprising a tangential slotdovetail.
 29. The steam turbine bucket airfoil claimed in claim 25,further comprising an axial entry dovetail.
 30. The steam turbine bucketairfoil claimed in claim 26, further comprising a tangential fir-treedovetail.
 31. The steam turbine bucket airfoil claimed in claim 26,further comprising a tangential slot dovetail.
 32. The steam turbinebucket airfoil claimed in claim 26, further comprising an axial entrydovetail.